Encapsulated electroluminescent phosphor and method for making same

ABSTRACT

Encapusalated electroluminescent phosphor particles and method for making same. The phosphor particles are encapsulated in a very thin oxide layer to protect them from aging due to moisture intrusion. The particles are encapsulated via a vapor phase hydrolysis reaction of oxide precursor materials at a temperature of between about 25° C. and about 170° C., preferably between about 100° C. and about 150° C. The resultant encapsulated particles exhibit a surprising combination of high initial luminescent brightness and high resistance to humidity-accelerated brightness decay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/912,786,filed Jul. 13, 1992, which is a division of application Ser. No.07/514,440, filed Apr. 25, 1990, now U.S. Pat. No. 5,156,885.

FIELD OF THE INVENTION

The present invention relates to electroluminescent phosphor particles,particularly encapsulated phosphor particles which exhibit strongmoisture resistance and high electroluminescent brightness. The presentinvention also relates to a method for making such encapsulated phosphorparticles.

BACKGROUND

Phosphor particles are used in a variety of applications such as flatpanel displays and decorations, cathode ray tubes, and fluorescentlighting fixtures. Luminescence or light emission by phosphor particlesmay be stimulated by application of heat (thermoluminescence), light(photoluminescence), high energy radiation (e.g., x-rays or e-beams), orelectric fields (electroluminescence).

Electroluminescent ("EL") phosphors are of particular commercialimportance. The luminescent brightness and "maintenance" of suchbrightness of such phosphors are two important criteria forcharacterizing phosphor particles. Luminescent brightness is typicallyreported as a quantity of light emitted by the subject phosphor whenexcited. When reported in absolute brightness, e.g., in foot-Lamberts("ft-L"), the conditions under which the phosphor is excited should alsobe reported as the absolute luminescent brightness of a given phosphortypically depends upon a combination of several factors. For instance,the absolute brightness of an electroluminescent phosphor should bereported with specified voltage and frequency of the applied electricfield and temperature of the phosphor. The luminescent brightnessattained is also dependent in part upon the physical characteristics andspecifications of the test device used to measure the magnitude ofemitted light. A typical test device possesses many of the same membersas the thick film electroluminescent devices discussed below. Withregard to accurately determining the absolute brightness of a subjectphosphor, important characteristics thereof include the thickness of thephosphor layer, the concentration or loading of the phosphor particlesin the dielectric matrix, the characteristics of the particulardielectric matrix material, and the transparency of the front electrode.Because of the sensitivity of phosphor emission brightness to varyingconditions of excitement, the brightness of phosphors are more typicallyreported as relative brightnesses rather than as absolute brightness."Maintenance" refers to the rate at which phosphors lose brightnessduring operation. As discussed, by Thornton in ElectroluminescentMaintenance, Jour. of Electrochem. Soc., pp 895-907, Vol. 107, No. 11,November 1960, such a decrease in brightness with operating time is atypical characteristic of phosphors. Furthermore, the rate of decay issubstantially increased if the phosphor particles are subjected toconditions of high humidity while being operated. "Atmospheric watervapor is perhaps the most important adverse influence onelectroluminescence maintenance from the point of view of practicalapplication." Ibid. This effect of moisture or high humidity is referredto herein as "humidity-accelerated decay".

Decay characteristics observed during operation at zero relativehumidity are referred to as the intrinsic maintenance characteristics orintrinsic decay of the subject phosphor. The intrinsic decay varies withoperating conditions such as voltage, frequency, and temperature, but isessentially reproducible for a given phosphor for a given set ofoperating conditions. As noted by Thornton, operation in high humidity,e.g., relative humidity of greater than about 80 percent can increasethe decay rate by a factor of 10 or more with respect to the subjectphosphor's intrinsic decay.

Particulate EL phosphors are most commonly used in thick filmconstructions. These devices typically include a layer of an organicdielectric matrix, e.g., polyester, polyethylene terephthalate,cellulosic materials, etc., preferably having a high dielectricconstant, loaded with phosphor particles, e.g., sulfide-based phosphorparticles. Such layers are typically coated on a plastic substratehaving a transparent front electrode. A rear electrode, e.g., analuminum foil or screen printed silver ink, is typically applied to theback side of the phosphor layer. When an electric field is appliedacross the electrodes, the proximate portions of the layer emit light asthe phosphor particles therein are excited. Such constructions mayfurther comprise optional dielectric layers between the phosphor layerand rear electrodes.

Organic matrices and coatings can temporarily delay or slow the rate ofhumidity-accelerated decay, however, after moisture permeates the matrixor coating, rapid loss of luminescent brightness is typically exhibited.Organic matrices and substrate materials have typically beeninsufficiently effective in preventing diffusion of water vapor to thephosphor particles, and have accordingly been ineffective in preventingsubsequent decay of brightness. For this reason, thick filmelectroluminescent devices are typically encased in relatively thick,e.g., 25 to 125 microns, envelopes of moisture-resistant materials suchas fluorochlorocarbon polymers such as ACLAR Polymers from AlliedChemical. Some of the problems with such envelopes include typicallysubstantial expense, unlit borders, and potential for delamination,e.g., under heat.

U.S. Pat. No. 4,097,776 (Allinikov) discloses electroluminescentphosphors coated with a liquid crystal in a solution-based technique.U.S. Pat. No. 4,508,760 (Olson et al.) discloses encapsulation ofelectroluminescent phosphors via vacuum deposition of certain polymers.

It is also known to encapsulate phosphor particles in inorganiccoatings, e.g., oxide coatings. U.S. Pat. No. 3,264,133 (Brooks)discloses the deposition of coatings such as titania (TiO₂) on phosphorparticles by washing the particles in a predominantly alcohol solutionof a halogen-containing constituent, e.g., titanium tetrachloride, andthen drying and firing the particles.

Vapor phase reaction and deposition processes have been used to coatphosphor particles with inorganic coatings. Such techniques aretypically considered as superior in providing more complete, uniform,and defect-free coatings. Phosphor particles encapsulated with suchtechniques have exhibited substantial resistance to humidity-accelerateddecay. However, significant reductions in humidity-accelerated decay ofluminescent brightness have been obtained only in conjunction withgreatly diminished initial luminescent brightness and in some instances,undesirable color shift of the light emitted by the encapsulatedphosphor particles.

For instance, U.S. Pat. No. 4,855,189 (Simopoulous et al.) disclosesencapsulation of phosphor particles with SiO₂ via a chemical vapordeposition process ("CVD") wherein phosphor particles are subjected to atemperature of about 490° C. and an atmosphere of oxygen and silane gaswhile being agitated. Phosphor particles encapsulated in accordance withthis reference have been found to exhibit a substantial reduction ininitial electroluminescent brightness for given excitement conditions.

Air Force Technical Report AFFDL-TR-68-103 (Thompson et al., July 1968)discloses vapor phase encapsulation of electroluminescent phosphorparticles for the purpose of attempting to improve performance atelevated temperatures. That reference discloses use of a fluidized bedchemical vapor deposition ("CVD") process to deposit several differentoxide coatings onto zinc sulfide-based phosphors. Oxide coatings weredeposited from a variety of precursor materials at furnace settings ofabout 200° C. to about 500° C. The reactor temperature profile was suchthat the maximum temperature within the reaction zone was typically 100°C. higher than the nominal temperature setting, accordingly, the maximumtemperatures within the reactor ranged upward of about 300° C. for thevarious deposition runs disclosed therein. Titania-coated zincsulfide/zinc selenide phosphors were found to have a reducedhumidity-accelerated decay, but the initial luminescent brightness ofthe encapsulated phosphors was only about 25 percent that of theoriginal material in uncoated form.

The prior art does not disclose a technique for encapsulating phosphorparticles that provides desired moisture-resistance coupled with highlevels of initial luminescent brightness relative to the initialluminescent brightness of the uncoated phosphor particles.

SUMMARY OF THE INVENTION

The present invention provides novel encapsulated phosphor particleshaving thin, substantially transparent oxide coatings which exhibitunexpectedly high initial luminescent brightness coupled with surprisingresistance to humidity-accelerated decay of luminescent brightness. Thepresent invention also relates to a novel method for making suchencapsulated phosphor particles utilizing relatively low temperaturevapor phase hydrolysis reactions and deposition processes.

Briefly summarizing, encapsulated phosphor particles of the inventioneach comprise a particle of luminescent phosphor which is essentiallycompletely encapsulated within a substantially transparent, continuousoxide coating. In accordance with the invention, the encapsulatedparticle has an electroluminescent brightness which is equal to orgreater than 50 percent of the luminescent brightness of the originaluncoated phosphor particle when excited in the same manner. Further,encapsulated phosphor particles of the invention exhibit substantiallyreduced humidity-accelerated brightness decay, i.e., their brightnessdecay characteristics in operating conditions of 95 percent or morerelative humidity are substantially the same as their intrinsic decaycharacteristics, such that the percent of electroluminescent brightnessretained following 100 hours of operation in an environment having arelative humidity of at least 95 percent is greater than about 70percent, preferably greater than about 80 percent, and most preferablygreater than about 90 percent, of the intrinsic brightness of theencapsulated phosphor particles retained following 100 hours operationunder substantially the same operating conditions of temperature, andvoltage and frequency of applied electric field. Intrinsic brightness ofthe encapsulated phosphor particles refers to the electroluminescentbrightness of such particles when operated under a relative humidity ofless than 10 percent.

In brief summary, the novel method of the invention comprises:

a) providing an agitated bed of phosphor particles;

b) heating the bed to a temperature of between about 25° C. and about170° C.;

c) exposing the bed to one or more vapor phase oxide precursors suchthat the precursors chemically react and form hermetic, substantiallytransparent oxide coatings on the surfaces of the particles, therebyyielding essentially encapsulated phosphor particles; and

d) cooling the resultant encapsulated particles.

The initial electroluminescent brightness of encapsulated phosphorparticles of the invention is typically at least about 50 percent of theinitial luminescent brightness of the phosphor particles in theirinitial uncoated state, preferably at least about 70 percent of thatinitial brightness, and most preferably at least about 80 percent ofthat initial brightness. The brightness decay during operation whileexposed to high humidity, e.g., relative humidity of over 80 percent, ismuch less than that of the uncoated phosphor under the same conditions,and is typically substantially the same as the intrinsic decay of thesubject phosphor. Phosphor particles of the invention provide asurprising combination of high initial electroluminescent brightness andhumidity resistance, a combination which was heretofore unavailable.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further explained with reference to the drawing,wherein:

FIG. 1 is a schematic .illustration of one embodiment of the method formaking encapsulated phosphor particles in accordance with the presentinvention;

FIG. 2 is a cross-sectional illustration of phosphor particles of theinvention; and

FIGS. 3 and 4 are graphical illustrations of the decay characteristicsof illustrative encapsulated phosphor particles of the invention and thedecay characteristics of uncoated phosphor particles of the samecomposition.

These figures are idealized and are intended to be merely illustrativeand non-limiting.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Phosphor particles used in the invention comprise zinc sulfide-basedelectroluminescent materials. Such phosphors are well-known and commonlyinclude one or more of such compounds as copper sulfide (CuS), zincselenide (ZnSe), and cadmium sulfide (CdS) in solid solution within thezinc sulfide crystal structure or as second phases or domains within theparticle structure.

Phosphors commonly contain moderate amounts of other materials such asdopants, e.g., bromine, chlorine, manganese, silver, etc., as colorcenters, as activators, or to modify defects in the particle lattice tomodify properties of the phosphor as desired. Phosphors used in thepresent invention may be formulated in accordance with conventionalpractices.

Good results have been obtained with several commercially availablephosphors including Sylvania Type 723, 728, and 830 Phosphors. SylvaniaType 723 is believed to be a copper-activated zinc sulfide phosphorwhich provides green fluorescence under an applied electric field.Sylvania Type 728 is believed to be a copper-activated zinc sulfidephosphor which provides green fluorescence under an applied electricfield. Sylvania Type 830 is believed to be a blend ofcopper/manganese-activated zinc sulfide and copper-activated zincsulfide which provides a white fluorescence under an applied electricfield.

Phosphor particles used herein may be of many sizes, typically dependingto large extent on the particular application. Phosphor particles havingaverage particle diameters of between about 1 and about 50 microns,preferably between about 10 and 40 microns, are typically useful forscreen printed and roll coated panels, for CRT screens, light bulbs, aswell as many other applications. Phosphor particles which are too largemay interfere with formation of very thin phosphor layers, may result ingrainy or nonuniform light output, and typically tend to settle tooquickly from suspensions during device fabrication. Phosphor particleswhich are too small may tend to degrade more rapidly during use due toincreased relative surface area, may tend to agglomerate so as tointerfere with free flow characteristics, and may be difficult to mixwith binders in desirably high loadings.

Encapsulated phosphor particles of the invention are essentiallycompletely coated with a substantially continuous coating of one or moreoxides. As used herein, "oxide coating" means a material made upprimarily of metal cations and oxygen, but which may contain minoramounts of other elements and compounds originating in the precursormaterials or phosphor particles, which can be generated in coating formon phosphor particles under the conditions described herein.Advantageous results have been obtained with coatings of titania (TiO₂)and titania/silica TiO₂ /(SiO₂). It is believed that useful results mayalso be obtained with other oxides formed from precursors in lowtemperature reactions such as silica (SiO₂), alumina (Al₂ O₃), tin oxide(SnO₂), zirconia (ZrO₂), etc., and similarly formed compound oxides suchas mullite (3Al₂ O₃.2SiO₂).

The oxide coating is substantially transparent and is typically betweenabout 0.1 and 3.0 microns thick, preferably between about 0.1 and about0.5 microns thick. Coatings which are too thin may tend to provideinsufficient impermeability to moisture. Coatings which are too thickmay tend to be less transparent and result in reduced brightness.

As mentioned above, the novel method of the invention comprises:

a) providing an agitated bed of phosphor particles;

b) heating the bed to a temperature of between about 25° C. and about170° C., preferably between about 100° C. and about 150° C;

c) exposing the bed to one or more vapor phase oxide precursors suchthat the precursors chemically react to form oxides in the presence ofthe phosphor particles and deposit on the surfaces of the phosphorparticles an essentially continuous, substantially transparent oxidecoating, thereby yielding essentially encapsulated phosphor particles;and

d) cooling the resultant encapsulated particles.

An illustrative embodiment of the novel process of the invention isshown schematically in FIG. 1.

During manufacture, phosphor particles may typically be washed to removeresidual amounts of dopants left on the surfaces thereof, e.g., coppersulfide may be removed with a potassium cyanide solution. Generally,commercially available phosphor particles are suitable for use in thepresent invention in the condition supplied by the manufacturer withoutany further special surface preparation.

Uncoated phosphor particles 12 are placed in reactor 14 and heated tobetween about 25° C. and about 170° C., preferably between about 100° C.and 150° C. In order to form substantially continuous coatings coveringessentially the entire surfaces of the phosphor particles, the particlesare preferably agitated while in the reaction chamber. Illustrativeexamples of useful methods for agitating the phosphor particles includeshaking, vibrating, or rotating the reactor, stirring the particles, orsuspending them in a fluidized bed. In such reaction chambers, theparticles may be agitated by many different ways such that essentiallythe entire surface of each particle is exposed and the particles andreaction precursors may be well intermixed. Typically, a preferredreaction chamber is a fluidized bed reactor. Fluidizing typically tendsto effectively prevent agglomeration of the particles, achieve uniformmixing of the particles and reaction precursor materials, and providemore uniform reaction conditions, thereby resulting in highly uniformencapsulation characteristics.

Although not required in many instances, it may be desired when usingphosphor particles which tend to agglomerate to add fluidizing aids,e.g., small amounts of fumed silica. Selection of such aids and ofuseful amounts thereof may be readily determined by those with ordinaryskill in the art.

Precursor materials in vapor phase are then added to the reactor. Thepresent invention utilizes a vapor phase hydrolysis reaction to form acoating of oxide material on the surfaces of the phosphor particlesthereby encapsulating them. Such process is sometimes referred to as achemical vapor deposition ("CVD") reaction. The following is anillustrative reaction:

    TiCl.sub.4 +4H.sub.2 O→TiO.sub.2 +2H.sub.2 O+4HCl

In the illustration, water vapor and titanium tetrachloride areconsidered oxide precursor materials.

One technique for getting the precursor materials into vapor phase andadding them to the reaction chamber is to bubble a stream of gas,preferably inert, referred to herein as a carrier gas, through asolution or neat liquid of the precursor material and then into thereaction chamber. Illustrative examples of inert gases which may be usedherein include argon and nitrogen. Oxygen and/or air may also be used.An advantage of this technique is that the carrier gas/precursor streamsmay be used to fluidize the phosphor particles in the reaction chamber,thereby facilitating the desired encapsulation process. In addition,such a technique provides means for readily controlling the rate ofintroduction of the precursor materials into the reactor. Referringagain to FIG. 1, carrier gas 2 is bubbled through water bubbler 4, toproduce water vapor-containing precursor stream 8, and carrier gas 2 isalso bubbled through titanium tetrachloride bubbler 6, to producetitanium tetrachloride-containing precursor stream 10. Precursor streams8 and 10 are then transported into reactor 14.

Precursor flow rates are adjusted to provide an adequate deposition rateand to provide an oxide coating of desired quality and character. Flowrates are adjusted such that the ratios of precursor materials presentin the reactor chamber promote oxide deposition at the surface of thephosphor particles substantially without formation of discrete, i.e.,free floating, oxide particles, elsewhere in the chamber. For example,when depositing coatings of titania from titanium tetrachloride andwater, a high ratio of tetrachloride molecules to water molecules ismaintained such that most of the available water in the reaction chamberremains absorbed on the surfaces of the phosphor particles and little isin free state elsewhere in the chamber. Such a ratio is also believed topromote the formation of more anhydrous titania films which are believedto provide optimum protection against humidity-accelerated decay.

Optimum flow rates for a particular application typically depend in partupon the temperature within the reaction chamber, the temperature of theprecursor streams, the degree of particle agitation within the chamber,and the particular precursors being used, but useful flow rates may bereadily determined with trial and error. In preferred embodiments, theflow rate of carrier gas used to transport the precursor materials tothe reaction chamber is sufficient to agitate the phosphor particles asdesired and also transport optimal quantities of precursor materials tothe chamber.

Preferably, the precursor materials have sufficiently high vaporpressures that sufficient quantities of precursor material will betransported into the reactor for the hydrolysis reaction and coatingprocess to proceed at a conveniently fast rate. For instance, precursormaterials having higher vapor pressures will typically provide fasterdeposition rates than will precursor materials having lower vaporpressures, thereby enabling the use of shorter encapsulation times.Precursor sources may be heated to increase the vapor pressure of thematerial, however, this may necessitate heating of tubing or other meansused to transport the precursor material to the reactor so as to preventcondensation between the source and the reactor. In many instances,precursor materials will be in the form of neat liquids at roomtemperature. In some instances, the precursor materials may be availableas sublimable solids.

Precursor materials that are capable of forming hermetic oxide coatingsvia hydrolysis reactions at low temperatures, e.g., below about 170° C.and preferably below about 150° C., are preferred. Advantageous resultshave been obtained with titanium tetrachloride or silicon tetrachloride,and water as precursor materials. In addition to such metal chlorides,useful results are also expected with metal alkoxides, e.g., titaniumisopropoxide, silicon ethoxide, and zirconium n-propoxide.

Preferably, the mutually reactive precursor materials, e.g., TiCl₄ andH₂ O, are not mixed prior to being added to the reactor in order toprevent premature reaction within the transport system. Accordingly,multiple gas streams into the reactor chamber are typically provided.

The temperature of the reactor is maintained at between about 25° C. andabout 170° C., and preferably between about 100° C. and about 150° C. Ithas been observed that encapsulation processes performed at temperatureswithin this range provide deposition of desired hermetic coatings thatprovide desired protection against humidity-accelerated decay whileavoiding intrinsic thermal damage or adverse thermochemical reactions atthe surfaces of the particles which cause undesirable loss of initialbrightness. Encapsulation processes which are performed at temperatureswhich are too low may tend to result coatings which do not providedesired resistance to humidity-accelerated decay. Such coatings are notsufficiently moisture impermeable, a result it is believed of having amore open or more hydrated structure. Encapsulation processes which areperformed at temperatures which are too high may result in decreasedelectroluminescent brightness, undesirable changes or shifts in thecolor of the light emitted by the subject phosphor, or degradation ofthe intrinsic decay characteristics of the subject phosphor material.

Although it has been suggested in the prior art that exposing phosphorparticles to high temperatures, e.g., above about 350° C., tends toreduce the initial luminescent brightness thereof, it has been foundthat phosphor particles may be degraded by exposure to lowertemperatures, e.g., about 170° to about 200° C., under certainconditions. While I do not wish to be bound by this theory, it ispostulated that phosphor materials are not sensitive only to thetemperatures to which they are exposed, but that one or more effectscaused by exposure of the particles to certain compositions, e.g.,exposure to certain compounds, also exist, and that such effects arealso dependent upon temperature. A specific mechanism is not yetdetermined, but it is believed that the surface of the phosphorparticles may undergo some change by exposure to such agents ashydrochloric acid such as is generated during vapor generation anddeposition of titania coatings from titanium tetrachloride which affectsthe luminescent brightness of the resultant encapsulated particle.

Accordingly, encapsulation of phosphor particles as described herein ispreferably performed at temperatures between about 25° C. and about 170°C., preferably between about 100° C. and about 150° C. Referring againto FIG. 1, following encapsulation, encapsulated phosphor particles 16of the invention are removed from reactor 14. As illustrated in FIG. 2,typically encapsulated phosphor particles 20 of the invention consistessentially of particle 22 of phosphor material which is essentiallycompletely encapsulated within substantially transparent, continuousoxide coating 24.

Encapsulated phosphor particles of the invention provide both highresistance to humidity-accelerated decay and substantially retain theirintrinsic properties. For instance, there is typically little or noshift in the emission spectra of phosphor particles encapsulated astaught herein, such particles typically retain a substantial portion oftheir initial luminescent brightness, and the intrinsic decaycharacteristics are typically similar to or even better than those ofthe uncoated phosphor particles.

The resistance to humidity-accelerated decay is typically such that therate of brightness loss when operated while directly exposed to highhumidity, e.g., a relative humidity of greater than 95 percent, issubstantially no greater than the intrinsic brightness loss exhibitedduring operation in a dry environment, e.g., a relative humidity of lessthan about 10 percent. In an illustrative example, the luminescentbrightness of an encapsulated phosphor of the invention, after operationfor 100 hours in an environment having a relative humidity of at least95 percent, was over 90 percent of the luminescent brightness ofsimilarly encapsulated phosphor particles after operation for 100 hoursin an environment having a relative humidity of less than 10 percent.

FIG. 3 is a graphical illustration of the relative absoluteelectroluminescent brightness versus time of operation of illustrativeencapsulated phosphor particles of the invention and the same phosphormaterial in uncoated state. In FIG. 3, the difference in position on theY (vertical) axis is proportional to the difference in absolutebrightness of the subject phosphors. Each curve was derived from theaverage of several samples of the indicated type. Curve 50 representsthe decay characteristics of uncoated phosphor material operated in adry environment (relative humidity less than 10 percent) and Curve 52represents the decay characteristics of uncoated phosphor materialoperated in a high humidity environment (relative humidity over 95percent). The substantial difference between Curve 50 and Curve 52represents humidity-accelerated brightness decay of the uncoatedphosphor material. Curve 60 represents the decay characteristics ofencapsulated phosphor particles of the same phosphor material,encapsulated in accordance with the invention, operated in the same dryenvironment. Curve 62 represents the decay characteristics Ofencapsulated phosphor particles of the same phosphor material,encapsulated in accordance with the invention, operated in the samehumid environment described above. The small differential between Curves60 and 62 indicates that humidity-accelerated brightness decay has beensubstantially eliminated by encapsulation in accordance with the presentinvention. Curves 60 and 62 begin at lower absolute brightness,representing the reduction in initial electroluminescent brightness(about 75 percent of that of the uncoated phosphor material) resultingfrom the encapsulation process. Such performance is substantially betterthan that achieved with previously known encapsulation techniques. Forinstance, phosphor particles encapsulated in accordance with U.S. Pat.No. 4,855,189 have been found to have an initial brightness of onlyabout 30 percent of that of the uncoated phosphor.

FIG. 4 is a graphical illustration of the percent of retainedluminescent brightness of each of the subject phosphors versus time ofoperation of encapsulated phosphor particles of the invention and thesame phosphor material in an uncoated state. Each curve was derived fromthe average of several of samples of the indicated type. Curve 54represents the decay characteristics of uncoated phosphor materialoperated in a dry environment (relative humidity less than 10 percent)and Curve 56 represents the decay characteristics of uncoated phosphormaterial operated in a high humidity environment (relative humidity over95 percent). Curve 64 represents the decay characteristics ofencapsulated phosphor particles of the same phosphor material,encapsulated in accordance with the invention, operated in the same dryenvironment. Curve 66 represents the decay characteristics ofencapsulated phosphor particles of the same phosphor material,encapsulated in accordance with the invention, operated in the samehumid environment.

In accordance with the present invention, encapsulated phosphorparticles may be made which exhibit the exceptional resistance tohumidity-accelerated decay described above and also provide high initialelectroluminescent brightness. For instance, encapsulated phosphorparticles of the invention can be made with Sylvania Type 723 Phosphorwhich exhibit an initial electroluminescent brightness of at least about15 foot-Lamberts, preferably at least about 20 foot-Lamberts, mostpreferably at least about 23 foot-Lamberts, as measured by providing a100 micron thick layer of the encapsulated phosphor particles indielectric oil (castor oil) with an ITO on glass electrode (transmissionabout 90 percent) that had been sprayed with substantially transparentacrylic coating about 1000 angstroms thick thereover, the layercontaining 66 weight percent of phosphor particles, applying an electricfield having a voltage of about 600 volts and a frequency of about 500Hertz, and measuring the magnitude of light emitted through theelectrode. In an uncoated state, Sylvania Type 723 Phosphor was found toexhibit an initial electroluminescent brightness of about 29.5foot-Lamberts. In an uncoated state, Sylvania Type 728 Phosphor has beenfound to exhibit an initial electroluminescent brightness of about 31.5foot-Lamberts, and encapsulated phosphor particles of the inventionhaving brightnesses of at least about 16, preferably 22, and mostpreferably 25 foot-Lamberts can be made therewith. In an uncoated state,Sylvania Type 830 Phosphor has been found to exhibit an initialelectroluminescent brightness of about 11 foot-Lamberts, andencapsulated phosphor particles of the invention having brightnesses ofat least about 6, preferably 8, and most preferably 9 foot-Lamberts canbe made therewith.

EXAMPLES

The invention will be further explained by the following illustrativeexamples which are intended to be nonlimiting. Unless otherwiseindicated, all amounts are expressed in parts by weight. Flow ratesrefer to the metered volume of carrier gas (nitrogen gas) through theindicated solutions.

Encapsulation Process

Fluidized bed reactors consisting of glass-frit type funnels with asingle bottom inlet and size D frit were used. As indicated below, 20millimeter and 40 millimeter reactors modified for oil bath immersion orfor heating with nichrome wire were used. The 20 millimeter reactorswere used with a single gas inlet tube and the 40 millimeter reactorswith two gas inlet tubes. The gas inlet tubes were glass tubes, 10millimeters in diameter, with size C glass frits which were insertedinto the fluidized bed extending from the top of the funnel to introducecarrier gas and metal tetrachloride vapors into the reaction zone. Aseparate tube was connected to the bottom of the reactor and water vaporintroduced into the reactor therethrough.

Bubbler sizes were about 300 milliliters for the 20 millimeter diameterreactors and 800 milliliters for the 40 millimeter diameter reactors.

Carrier gas and water vapor were passed through the funnel fritsupporting the phosphor particles. Reagent grade neat liquids oftitanium tetrachloride and silicon tetrachloride from Aldrich ChemicalCompany were used as indicated.

Brightness

The electroluminescent brightness of phosphor samples was determined intest cells comprising a machined aluminum grid with 100 micron spacingbetween electrodes. Each cell was filled with a mixture of phosphorparticles and liquid dielectric oil, Dow Corning FS1265 fluorosiliconoil or castor oil, at about 66 weight percent particles. A transparenttop electrode comprising a sheet of indium tin oxide coated polyesterfilm (DX ITO/PE from Southwall Corporation), having about 90 percenttransmission, was mounted over the top of the grid. Tests were run underan applied electric field of 220 volts, 400 Hertz, in sealed batteryjars maintained with water-saturated air, i.e., relative humidity ofabove 95 percent, or desiccant, i.e., relative humidity of below 10percent. Samples were run continuously for at least 96 hours.

Abbreviations

The following abbreviations are used in reporting the examples:

    ______________________________________                                        Abbrev.   Meaning                                                             ______________________________________                                        IB        Initial Brightness of phosphor sample at                                      beginning of brightness test as percentage of                                 initial luminescent brightness of same                                        phosphor in fresh, uncoated condition.                              RB        Retained Brightness of phosphor sample after                                  about 96 hours continuous operation of                                        brightness cell as percentage of Initial                                      Brightness of same phosphor.                                        RH        Relative Humidity under which luminescent                                     brightness was determined.                                          SEM       Scanning Electron Microscope.                                       ______________________________________                                    

Phosphor Specifications

Commercially available Sylvania type 723, 723RB, 728, and 830 Phosphorswere used in the Examples as indicated. The physical properties of thosephosphors are reported by the seller as follows:

    ______________________________________                                        Size Distrib..sup.1     Density.sup.2                                         Type  25%    50%     75%  SS.sup.3                                                                            M    B    Color.sup.4                         ______________________________________                                        723   22     28      35   22    4.1  1.94 Light green                         728   24     31      38   23    4.1  1.94 Light green                         830   22     28      35   22    4.1  1.94 Light tan                           ______________________________________                                         .sup.1 Particle Size Distribution  Coulter Counter, size in micrometers a     listed percentiles.                                                           .sup.2 Material ("M") and Bulk ("B") Density in grams/cubic centimeter.       .sup.3 Fisher SubSieve Size.                                                  .sup.4 Body color.                                                       

Type 723 RB Phosphor has been observed to exhibit a greater shift toblue emission when high frequency electric fields are applied, but isotherwise believed to be substantially similar to Type 723 Phosphor.Type 723RB Phosphor is reported to have the same physical properties aslisted above for Type 723.

Example 1

A 20 millimeter diameter reactor heated with nichrome wire was used toencapsulate 20 grams of Sylvania Type 723 Phosphor with titania.

During encapsulation the temperature was maintained at 137° C.±8° C. Theflow rates of dry nitrogen through the water and titanium bubblers were100 centimeters³ /minute and 260 centimeters³ /minute, respectively. Theencapsulation process was run for 4 hours. Small samples were removedfrom the reactor every hour and immersed in 0.1 molar silver nitratesolution and observed. The uncoated phosphor turned black within a fewminutes as silver sulfide formed at the surface of the particles.Phosphor particles removed after 1 hour turned gray, indicatingincompletely encapsulated particles. Phosphor particles removed after 2or more hours were unaffected by the solution, indicating that they wereessentially completely encapsulated with a coating which was impermeableto the solution. Negligible change in coloration of the immersedparticles was observed over a period of several weeks.

SEM analysis revealed that the phosphor particles had coatingthicknesses of between 0.2 and 0.4 microns. The coatings appeared tocompletely cover the surfaces of the particles and no pores werevisible.

Brightness results of the encapsulated phosphor particles, identified asSample 1, are tabulated in Table I below. The corresponding results foruntreated phosphor particles, identified as sample A are also listed forcomparison.

                  TABLE I                                                         ______________________________________                                        Sample   RH             IB     RB                                             ______________________________________                                        A        <10            100    75                                             A        >95            100     0                                             1        <10             77    88                                             1        >95             77    88                                             ______________________________________                                    

The Retained Brightness of Sample A differed markedly between operationin humid conditions and operation in dry conditions. However, in Sample1 it was substantially the same, indicating high resistance tohumidity-accelerated decay. In other tests, operation of encapsulatedphosphor particles of the invention in humid environments was found tohave resulted in a somewhat lower Retained Brightness than operation indry environments, but in all instances the differential was small andhigh resistance to humidity-accelerated decay was obtained.

Examples 2-7

Several 20 gram batches of Sylvania No. 723 Phosphor were coated withtitania as in Example 1, except the average temperature and flow rateswere varied as indicated. The reaction conditions and brightnessproperties of the resultant encapsulated phosphor particles aretabulated in Table II.

                  TABLE II                                                        ______________________________________                                        Sample  Temp.sup.1                                                                              Water.sup.2                                                                           TiCl.sub.4.sup.3                                                                       IB  RB                                     ______________________________________                                        2       130       100     220      77  85                                     3       140       100     220      73  82                                     4       150       100     220      69  59                                     5       140       200     120      72  18                                     6       140       170     120      68  56                                     7       140       120     200      73  86                                     ______________________________________                                         .sup.1 Average reaction temperature in °C.                             .sup.2 Flow rate through water bubbler in centimeters.sup.3 /minute.          .sup.3 Flow rate through TiCl.sub.4 bubbler in centimeters.sup.3 /minute.

Examples 2-4 illustrate a tendency toward reduced initial luminescentbrightness with increasing reaction temperature, indicating thatminimization of reaction temperature below certain levels is importantfor maintaining high initial luminescent brightness.

Examples 5-7 illustrate a tendency toward increased retention ofluminescent brightness with higher ratios of titanium tetrachloride towater precursor flows. This effect may have been observed because theresultant coatings made with lower ratios were less anhydrous or becausethe limited amount of available titanium tetrachloride resulted inslower reaction and thinner resultant coating, which in thicker formmight have provided better resistance to humidity-accelerated decay.

Examples 8-10

Several batches of Sylvania No. 723 Phosphor were encapsulated as inExample 1 except (1) an oil bath was used to maintain and averagereaction temperature of about 140° C.±5° C. and (2) the chloride bubblercontained a mixture of titanium tetrachloride and silicon tetrachloridein the indicated volume ratio. The flow rate through the water bubblerwas 100 centimeters³ /minute and the flow rate through the chloridebubbler was 220 centimeters³ /minute.

                  TABLE III                                                       ______________________________________                                        Sample   Ratio           IB    RB                                             ______________________________________                                        8        40/60           82    89                                             9        60/40           87    88                                             10       80/20           80    83                                             ______________________________________                                    

Examples 11-15

Several batches of Sylvania No. 723 Phosphor were encapsulated as inExample 1 except that a 40 millimeter diameter reactor and 100 grams or200 grams as indicated of phosphor were used, and the reaction wascontinued for the indicated time. Nitrogen flow rates through the waterand titanium tetrachloride bubblers were 1300 cubic centimeters/minuteand 350 cubic centimeters/minute, respectively.

                  TABLE IV                                                        ______________________________________                                        Sample    Temp.sup.1                                                                            Amount.sup.2 Time.sup.3                                                                          IB                                       ______________________________________                                        11        200     200          9     14                                       12        187     100          7     22                                       13        170     100          5     32                                       14        157     100          5     60                                       15        150     100          5     67                                       ______________________________________                                         .sup.1 Average reaction temperature in °C.                             .sup.2 Amount of phosphor in charge.                                          .sup.3 Length of reaction time.                                          

Examples 11-15 illustrate a tendency toward reduced initial luminescentbrightness with increasing reaction temperature, indicating thatminimization of reaction temperature below certain levels is importantfor maintaining high initial luminescent brightness.

Examples 16-25

Several 150 gram batches of Sylvania No. 723 Phosphor were encapsulatedwith titania using a 40 millimeter diameter reactor with 2 top gasinlets for oxide precursors. The temperature was controlled to +2° C. ofthe indicated value using an oil bath.

                  TABLE V                                                         ______________________________________                                        Sample                                                                              Temp.sup.1                                                                            A Flow.sup.2                                                                           B Flow.sup.3                                                                         Water.sup.4                                                                         Time.sup.5                                                                          IB  RB                              ______________________________________                                        16    135     600      600    600   5.5   77  87                              17    135     600      600    600   6.0   74  82                              18    138     640      600    310   7.5   75  77                              19    138     640      600    350   7.5   77  73                              20    138     640      600    450   8.5   72  81                              21    128     640      600    450   7.5   71  80                              22    128     640      600    450   8.0   69  83                              23    128     640      600    480   8.0   72  88                              24    138     680      700    550   4.0   84  81                              25    138     680      700    600   4.0   79  88                              ______________________________________                                         .sup.1 Reaction temperature in °C.                                     .sup.2 Flow rate through TiCl.sub.4 bubbler A in centimeters.sup.3            /minute.                                                                      .sup.3 Flow rate through TiCl.sub.4 bubbler B in centimeters.sup.3            /minute.                                                                      .sup.4 Flow rate through water bubbler in centimeters.sup.3 /minute.          .sup.5 Reaction time in hours.                                           

Examples 26-28

Three 20 gram batches of encapsulated phosphor particles were made usingSylvania No. 723, 728, and 830 Phosphor, respectively. In each case thephosphor particles were encapsulated using a 20 millimeter diameterreactor heated in an oil bath to an average temperature of about 128° C.The encapsulation reaction was run for 3.5 hours at the indicated flowrates.

                  TABLE VI                                                        ______________________________________                                        Sample   Phosphor  TiCl.sub.4.sup.1                                                                      Water.sup.2                                                                           IB  RB                                     ______________________________________                                        26       723RB     200     105     72  83                                     27       728       220     110     68  83                                     28       830       220     110     84  81                                     ______________________________________                                         .sup.1 Flow rate through TiCl.sub.4 bubbler in centimeters.sup.3 /minute.     .sup.2 Flow rate through water bubbler in centimeters.sup.3 /minute.     

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention.

What is claimed is:
 1. A process for encapsulating phosphor particles insubstantially transparent oxide coatings comprising:a) providing anagitated bed of zinc sulfide-based electroluminescent phosphorparticles; b) heating said bed; c) exposing said bed to one or morevapor phase metal oxide precursors such that said precursors chemicallyreact substantially via hydrolysis to form hermetic, substantiallytransparent oxide coatings on the surfaces of said particles, therebyyielding essentially encapsulated phosphor particles; and d) coolingsaid encapsulated particles.
 2. The process of claim 1 wherein said bedis heated to a temperature of between about 100° C. and about 150° C. 3.The process of claim 1 wherein said vapor phase oxide precursorscomprise water vapor and titanium tetrachloride.
 4. The process of claim1 wherein said vapor phase oxide precursors comprise water vapor,titanium tetrachloride, and silicon tetrachloride.
 5. The process ofclaim 1 wherein said vapor phase oxide precursors comprise water vaporand one or more metal alkoxides or metal chlorides.
 6. The process ofclaim 1 wherein said particles contain minor amounts of one or more ofthe following: cadmium, selenium, copper, bromine, chlorine, manganese,or silver.
 7. The process of claim 1 wherein said phosphor particles arebetween about 1 and about 50 microns in diameter.
 8. The process ofclaim 1 wherein said phosphor particle are between about 10 and about 40microns in diameter.
 9. The process of claim 1 wherein said oxidecoatings are between about 0.1 and about 3.0 microns thick.
 10. Theprocess of claim 1 wherein said oxide coatings are between about 0.1 andabout 0.5 microns thick.
 11. The process of claim 1 wherein said bed isheated to a temperature of between about 25° C. and about 170° C.
 12. Aprocess for encapsulating phosphor particles in substantiallytransparent oxide coatings, comprising:a) providing an agitated bed ofzinc sulfide-based electroluminescent phosphor particles; b) heatingsaid bed to a temperature that does not substantially degrade thephosphor particles; c) exposing said bed to one or more vapor phasemetal oxide precursors such that said precursors chemically reactsubstantially via hydrolysis to form hermetic, substantially transparentoxide coatings on the surfaces of said particles, thereby yieldingessentially encapsulated phosphor particles; and d) cooling saidencapsulated particles.
 13. The process of claim 12 wherein the vaporphase oxide precursors do not generate hydrochloric acid during vaporgeneration and deposition of the oxide coatings when said temperatureexceeds 170° C.
 14. A process for encapsulating phosphor particles insubstantially transparent oxide coatings, comprising:a) providing anagitated bed of zinc sulfide-based electroluminescent phosphorparticles; b) heating said bed to a temperature that does notsubstantially degrade the phosphor particles as a result of intrinsicthermal damage or adverse thermochemical reactions at the surface of thephosphor particles; c) exposing said bed to one or more vapor phasemetal oxide precursors such that said precursors chemically reactsubstantially via hydrolysis to form hermetic, substantially transparentoxide coatings on the surfaces of said particles without generatinghydrochloric acid, thereby yielding essentially encapsulated phosphorparticles; and d) cooling said encapsulated particles.